Arcing in microwave ovens is caused by metal. If there is no metal, there is nothing to arc.
If you see arcing in a food item, there has to be metal in that item.
Could the container that you're melting the butter in contain some metal? Gold or silver foil? Metallic paint? Could the arcing you're seeing actually be coming from something other than the butter?
Sputtering is another matter. Can be caused by pockets of superheated oil, water, etc.